Salt and wealth

George Bush is reading the book Salt: a world history by Mark Kurlansky and the author is quoted as saying: "he reads books?"
I was surprised to learn that you can write a whole book about salt. I was further surprised by Kurlansky arguments. According to this book, Salt was as valuable as gold mainly for its preservative qualities. Therefore, International trade could not take place without salt before refrigeration.
It got me thinking could this be used to illustrate shift in Supply and Demand?
Why is salt not valuable in the 20th century?
According to this book Salt in the 2oth century became abundant because of drilling technology. (Change or shift in supply)
Also refrigeration seems to have reduced the need for salt. (Change or shift in Demand)
As Salt becomes abundant Diminishing Marginal utility sets in? Similar to the Diamond water paradox? ( I have not worked this out completely)